1. Field of the Invention
With the increasing prevalence of renewable energy generators, such as off-shore wind, there is a growing need to transmit direct current (DC) electrical power from one high voltage DC power transmission network to another high voltage DC power transmission network.
2. Description of Related Art
Such transmission between high voltage DC networks can be achieved through the provision of first and second conventional voltage converters 10, 12 which are interconnected by a high power transformer 14, as shown in FIG. 1.
In a first mode of operation the first voltage converter 10 converts an incoming DC voltage from a first high voltage DC network 16 into an alternating voltage which the transformer 14 increases or decreases to allow conversion by the second voltage converter 12 to a desired outgoing DC voltage, as required in a second high voltage DC network 18.
In a second mode of operation the second voltage converter 12 converts an incoming DC voltage from the second high voltage DC network 18 into an alternating voltage which the transformer 14 increases or decreases to allow conversion by the first voltage converter 10 to a desired outgoing DC voltage, as required in the first high voltage DC network 16.
Such first and second conventional voltage converters 10, 12 provide a wide range of functionality and so are deployable in a large number of DC power transmission scenarios. However, they require two fully rated power electronic converters, i.e. conventional voltage converters 10, 12, and an intermediate transformer 14, meaning they tend to be large, heavy, and expensive.
There is, therefore, a need for a small, lightweight, inexpensive, and reliable means of connecting first and second high voltage DC power transmission networks.
According to an aspect of the invention there is provided a DC to DC converter assembly, for connecting first and second high voltage DC power transmission networks, comprising:
an inverter having first and second terminals connectable in use to a first high voltage DC power transmission network, the inverter being defined by a modular multilevel converter including a first inverter limb extending between the first and second terminals and having first and second inverter limb portions separated by a third terminal, each inverter limb portion including at least one rationalised module having first and second sets of series-connected current flow control elements connected in parallel with at least one energy storage device, each set of current flow control elements including an active switching element to selectively direct current through the energy storage device and a passive current check element to limit current flow through the rationalised module to a single direction, the current flow control elements and the or each energy storage device combining to selectively provide a voltage source to synthesise an AC voltage at the third terminal; and                a rectifier electrically connected to the third terminal of the inverter by a first link and connectable in use to a second high voltage DC power transmission network, the rectifier being configured to convert the AC voltage conveyed by the first link into a second DC voltage for supply to the second high voltage DC power transmission network.        